drm/ttm: fix type mismatch error on sparc64

On sparc64, __fls() returns an "int", but the drm TTM code expected it
to be "unsigned long" as on x86.  As a result, on sparc (and arc, and
m68k) you get build errors because 'min()' checks that the types match.

As suggested by Linus, it can use min_t instead of min to force the type
to be "unsigned int".

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>